TM 1-1510-218-10 3A-29 (7)   FLT/ET    Button.        If    ET    is    currently displayed, pressing the  FLT / ET button will cause the FT  to  be  displayed.    Pressing  this  button  again  will exchange the two timers in the display.  If the standby frequency  is  displayed,  pressing  the   FLT  /  ET button will cause the timer that was last displayed to reappear in the window. NOTE When power is first applied the flight timer is displayed. (a)  Flight    Timer.        The    flight    timer    is displayed  in  the  right  portion  of  the  display  window when   the   FLT   message   is   displayed.      The   timer receives power through the landing gear squat switch.   The counter will count up to 59 hours, 59 seconds.  It begins counting upon liftoff and will quit counting upon landing.  The elapsed timer may be reset back to 0 by pressing the SET / RST button or turning the unit off. (b)  Elapsed Timer.   This coupler has two modes:  count  up  and  count  down.    When  power  is applied, it is in the count up mode starting at 0.  The timer will count up to 59 hours, 59 seconds displaying minutes  and  seconds  until  1  hour   has  elapsed,  then displaying hours and minutes.  When in the count up mode, the timer may be reset to 0 by pressing the  SET / RST button. NOTE Pressing   the   reset   button   will   reset   the elapsed timer regardless of what is currently being displayed. To enter  the count down mode, press the  SET / RST  button  for  approximately  2  seconds  until  the  ET message  begins  to  flash.    While  the  ET  message  is flashing, the timer is in the ET set mode.  In this mode a number up to 59 minutes, 59 seconds may be preset into the elapsed timer with the frequency select knobs.   With the smaller knob pressed in, the 10's of seconds digit may be changed; it will roll over from 5 to 0 and under  0  to  5.    With  the  knob  pulled  out,  the  1's  of seconds digit may be changed.  It rolls over  for 9 to 0 and under from 0 to 9.  The larger knob modifies the minutes.  It rolls over from 59 to 0 and under from 0 to 59.    The  timer  will  remain  in  the  ET  set  mode  (ET message  flashing)  for  15  seconds  after  a  number  is set in or until the SET / RST, FLT / ET, or FRQ button is pressed.  The preset number will remain unchanged until the SET / RST button is pressed.  When the  SET /   RST   button   is   pressed   after   a   number   has   been preset,  the  elapsed  timer  will  start  counting  down.   When the elapsed timer is counting down, pressing the SET / RST button again will have no effect unless it is held for approximately 2 seconds.  This will cause the timer  to  stop  and  enter  the  set  mode  (ET  message flashing).  When the timer reaches 0, it changes to the count up mode  and continues up from 0.  The elapsed time   will   flash   for   15   seconds,   then   annunciate   a steady display, (8)   FRQ  Button.    The  selected  frequency  is put into the active window by pressing the  FRQ button.   The standby and active frequencies will be exchanged. (9)   BFO  Button.    When the   BFO  button  is pressed,   the   BFO   mode   is   activated   and   BFO   will display in the display window.  In various parts of the world,  some  L/LM  stations  use  an  interrupted  carrier for  identification  purposes.    When  in  the  BFO  mode, these stations are more easily identified.  Pressing the BFO  button  will  cause  a  1000  Hz  tone  to  be  heard whenever there is a radio carrier signal present at the selected frequency. (10)   ADF  Button.    When the   ADF  button  is pressed, the unit is placed into the ADF mode with the loop  antenna  enabled.    An  ADF  message  on  the  left portion  of  the  window  will  display.    Indicator  needles, as   selected   on   the   RMI's,   will   point   to   the   relative bearing of the selected station. d.  Operating Procedure. 1.   AVIONICS MASTER Switch – On. 2.   OFF     /     VOL     Control     Switch – Turn clockwise out of detent.  Adjust volume as required. NOTE An  audio  muting  feature  causes  the  audio output  to  be  muted  unless  the  receiver  is locked  onto  a  valid  station.    This  reduces interstation  noise  and  aids  in  identifying navigable stations.  3.   ADF   Test   –   Select   ANT   mode.      Verify bearing   pointers   park   in   the   90-degree position. 4.   Operating Mode – Select as desired. 5.   Frequencies   –   Select.      Ensure   desired active frequency is displayed. 6.   FLT / ET Switch – Operate as required. e.   Shutdown Procedures.  1.   OFF / VOL Switch – Turn counterclockwise to off (detent). 2.   AVIONICS MASTER Switch – OFF.